What device do I need to check how much charge is left on my Rx batteries (which are typically buried in the fuse but with an accessible charge jack)?

Expanded scale volt meter with load capability. http://www3.towerhobbies.com/cgi-bin...&I=LXWW16&P=ML
but there are others out there.

I've been using this one for years:
http://www.horizonhobby.com/Products...source=froogle

What does the 1A, 2A, and 5A stand for?

I think it is .5A, 1A, 2A; its the load so the load selection is 500ma, 1000ma, and 2000ma. Putting the load on the battery lets you measure it while it is trying to work. You are looking to see if the battery can hold its voltage while it is working, so when you use it, you should see no less than 4.8v for a 4 cell or 6.0v for a 5 cell nicad or nimh because at that point it is at the bottom of the charge. Anything above is good.

So how much load should I be placing on it? I'm strongly considering putting in a 5-cell battery.

It depends on the capacity of the pack. But the setting is usually about 1/3 to 1/2 of the capacity of the pack tested or closest choice.

I just ordered the Hanger 9 voltmeter. I like the permanently attached universal connector. It'll be nice to actually know for a change how much battery power I have left (and whether I've been silly flying 70 minutes on a single battery 4cell 1000aHm pack, Futaba NR-4RB).